1. Field of the Invention
This invention relates generally to automotive power steering systems and, more particularly, to variable effort power steering systems.
2. Description of the Prior Art
In typical power steering gears for automobiles, a valve which controls pressurization of the steering assist fluid motor is actuated by relative angular displacement between a spool rotatable with an input member of the steering gear and a sleeve rotatable with an output member of the steering gear. A torsion bar between the input and output members centers the sleeve and spool and resists such relative angular displacement between the input and output members. The resistance of the torsion bar is sensed by the driver as "road feel". The steering assist pressure in the fluid motor in such typical gears, referred to herein as boost pressure, increases exponentially relative to the increase in torsion bar resistance.
Because some drivers prefer more road feel at highway speeds than at zero or low vehicle speed, detent-type variable effort systems have been proposed wherein detents on the output member are urged by a detent fluid pressure into grooves in the input member. The detents connect the input member to the output member for unitary rotation until the handwheel effort applied by the driver exceeds a threshold magnitude proportional to the detent pressure at which instant the detents are cammed or ramped out of the grooves and the input member is released for rotation relative to the output member. Detent pressure typically increases with vehicle speed so that the threshold handwheel effort likewise increases with vehicle speed thereby yielding increased road feel at highway speeds. However, once the detents are ramped out of the grooves, boost pressure again increases exponentially relative to handwheel effort so that the driver may experience an abrupt change in the steering characteristic of the vehicle. A variable effort steering system according to this invention is a novel combination of a detent type variable effort system and a boost pressure modulation system which yields a substantially linear relationship between boost pressure and handwheel effort after the threshold handwheel effort is exceeded.